1. Field of the Invention
The present invention relates to a chemical vapor deposition (CVD), and more particularly, pertains to a method and apparatus for metal oxide chemical vapor deposition (MOCVD) upon a substrate surface.
2. Description of the Prior Art
Several difficulties in prior art metal oxide chemical vapor deposition have been found to be prevalent in the deposition of metal oxide chemical vapors upon a substrate, such as a wafer utilized for micro-chip manufacturing or other purposes. Difficulties encountered during deposition are mostly created by (a) undesirable topology on the substrate surface, (b) non-uniform heating of the substrate, and (c) a non-uniform gas boundary layer along and about the substrate.
The first problem in MOCVD is related to undesirable and inherent substrate topology where at low pressures, such as 1 Torr at which MOCVD best takes place, the different vapor components necessary to thermally decompose on a wafer surface in perfect stoichiometry for a particularly useful compound must arrive at the wafer surface at the correct ratio. If the wafer surface were always flat, this ratio could be achieved by simply altering the relative mixture of the vapors, i.e., 10/90 percent to 11/89 percent. However, in useful cases in integrated circuit construction, inherent uneven topology is usually present, i.e., 0.50 micron high plateaus with 0.25 micron spaces. In this case, each vapor component must arrive not only at the top of the topology features at the correct ratio, but must also arrive at the bottom of the topology features at the same ratio. If the deposition ratio is not maintained, then the composition of the complex metal oxide will be non-uniform and will not be useful. Since a gas boundary layer is usually present for a chemical vapor deposition (CVD) reactor with flowing gases of about 0.80 cm at 760 Torr, for example, for an 8-inch wafer b1=2/3L(v/UL) 0.50, each vapor component must replenish the boundary layer. Furthermore, different depths within the boundary layer must get the same ratio of vapors in order to allow uniform compositions to form from thermal decomposition. Generally, different metallorganic vapors diffuse at different speeds, as do most gases in general. The basic problem with the current art and prior art is that on uneven topography, uniform compound electrical and crystalline control is difficult, at best. Since the use of the dozens of new complex metal oxides will become prevalent, it is important to develop production methods to deposit the compounds that have been studied in planar applications.
The solution to the uneven topology problem, such as presented by the present invention, is to artificially reduce the boundary layer to significantly smaller and uniform thickness, such as in microns. The boundary layer thickness can be significantly reduced and the boundary layer uniformity can be enhanced and stabilized by any one method or combinations of methods including the use of an externally generated periodically disturbed gas motion in the form of a pressure wave, or by moving or oscillating the wafer itself, or by changing the pressure of the injected gas, any or all in the range of Hz through kilo Hz. With reference to reducing boundary layer thickness, Appendix A is attached. For example, b1=(v/pi.fr.d) where v=viscosity, pi is 3.14, fr=frequency, and d=density of the gas. Appendix A is a spreadsheet relating to various fluids and gases over an 8-inch wafer with no disturbance, and with either 40,000 Hz for water or 10 Hz for the N.sub.2 and argon, where the change in boundary layer (b1) is orders of magnitude.
If the boundary layer is made small, then the compound variation due to differential diffusion lengths will also be small, thereby offering a solution for the problem of two and three chemical component MOCVD. An additional benefit is the speeding up of the deposition rate, since most MOCVD reactions are limited by the delivery to the surface through a thick boundary layer.
The second and third prior art problems in MOCVD are the creation of uniform heating and a uniform gas boundary layer of any thickness. Improved uniformity of the gas layer boundary is accomplished in the present invention in part or wholly as previously described. Usually, a prior art rotating wafer in a downflow creates a uniform boundary layer, independent of scale. The speed of rotation controls, to a certain extent, the thickness. Physical rotation is limited by a vacuum rotating seal and particle problems in prior art devices. Rotation is also helpful or necessary in creating uniform lamp heating and is the subject of several existing patents, such as in Applied Materials, etc. If a prior art stationary platen is used to heat the wafer uniformly, then the gas boundary layer will be non-uniform from the center to the edge, with the center being thicker and the edge being a thinner boundary layer, or the gradient will either increase or decrease from left to right accordingly. The preferred embodiment of the present invention provides for crossflow longitudinally and laterally along the wafer structure. If prior art rotation is used to make the boundary layer uniform, then a rotating vacuum seal is necessary, and lamp heating is necessary which is inherently non-uniform due to re-radiation differences at the edges of the wafer where radiation emits from all sides instead of just one side. Usually prior art multiple heat zones and multiple pyrometer feedback zones are used to compensate for non-uniformities. As well as gas delivery uniformity, temperature drives the reaction, so temperature uniformity is critical. The present invention eliminates the need for pyrometry since it is non-rotating and allows the use of thermocouples embedded in a heated chuck. Multiple heat zones are eliminated since a large mass, thermally conductive heated chuck is used.
In the present invention, the new boundary layer created by the periodical disturbance of gas motion stabilizes the boundary layer thickness and reduces the usual thick boundary layer to a mere fraction; and gas delivery and temperature uniformity are achieved utilizing simple reactor construction. The vapors or gases are sent into the interior chamber of the reactor with associated pressure waves transmitted by a transducer, by a vibrating or oscillating chuck, or by other suitable devices.